The pharmacokinetics (PK) of isepamicin were studied in 8 febrile neutropenic patients with hematologic malignancy and in 20 young women with acute pelvic inflammatory disease (PID). Isepamicin was given as a slow intravenous infusion over 30 min at a dose of 15 mg/kg once daily (OD). Serum levels of isepamicin were determined by fluorescence polarization immunoassay, and PK analyses were obtained based on a one-compartment open model after 24 hours (steady state) and after 7 days. On day 1, the volume of distribution (Vd) of isepamicin, for both populations, appeared about 30% higher than classically reported in healthy individuals: 0.31 and 0.36 L/kg for neutropenic and PID patients respectively. However on day 7, Vd displayed significant reduction (0.28 and 0.27 L/kg, respectively for neutropenic and PID patients). A reduction of isepamicin clearance was also observed between day 1 and day 7 (137 vs 120 mL/min and 130 vs 101 mL/min for neutropenic and PID populations, respectively). Such changes are consistent with a significant increase in the Cmax concentrations (45 vs 50 mg/L, and 38 vs 49 mg/L) and in the AUC (136 vs 158 and 137 vs 162 mg/L.h) observed after a week of treatment in neutropenic and PID patients, respectively. In conclusion, taking into account the importance of reaching early active concentrations, we recommend the use of higher loading dose of isepamicin (>15 mg/kg) in neutropenic cancer patients and in women with PID, particularly in case of a combination with a possibly ineffective antibacterial agent, in case of infection with bacteria at upper limit of susceptibility, in the presence of high infectious inoculum or in the presence of sequestered sites of infection.